Last year, I did nine interview weekends and I agree with Extra Expresso, there is simply not enough time to recover between weekends because by the time you are home and settled for a day, you need to start preparing for the next weekend. Personally, I would filter out places once you find one or two places that seem like a really good fit. I went to all of them because I didn't want to leave any stones un-turned and didn't want to have any regrets about the process. As mentioned above, the weekends are a lot of fun so just enjoy them!
Northwestern's weekend is an interview, but it felt more like a recruitment weekend (not sure on acceptance rates, etc.). The interviews themselves were somewhat casual, only 2/5 interviews were with faculty...at least in the drug delivery/tissue engineering area.
Minnesota's skype interview isn't a technical interview. My interviewer briefly asked me about my research experiences, then proceeded to ask a series of questions regarding why PhD, why Minnesota, what are your research interests, etc. I wouldn't sweat the interview, good luck!